1. Field of the Invention
The present invention relates to an epoxy resin composition, and a prepreg and a printed wiring board using the epoxy resin composition, and more particularly to an epoxy resin composition including a composite curing agent, and a prepreg impregnated with such a composition and a printed circuit board (PCB) manufactured by using such a prepreg.
2. The Prior Arts
The epoxy resins are currently employed in the manufacture of the structural substrates of the printed circuit boards. In the manufacture of the printed circuit boards, the epoxy resins were first reacted with the curing agents, the curing accelerators and the solvents to obtain a mixture, called varnish. Then, a substrate (e.g. a reinforcing material) was impregnated with the varnish, followed by drying to obtain a prepreg. Then, a particular number of the prepregs were laminated, and a metal foil was additionally placed on at least one outermost layer of the laminate, and subsequently a particular circuit pattern was formed on the metal-clad laminate by etching the metal foil thereon. The manufactured printed circuit board must meet the criteria for heat resistance, dielectric properties, and chemical resistance,
The epoxy resins employed in the manufacture of the structural substrates of the printed circuit boards usually have high adhesive, mechanical and electrical insulation properties along with good chemical resistance. However, the main drawbacks to the epoxy resins are in brittleness and high water absorption. The epoxy resin can be cross-linked with a multi-functional amine to form an interpenetrating polymeric network (IPN). Although IPNs have favorable properties such as high glass transition temperature and high chemical resistance, they have the drawback of being too brittle to be processed as prepregs. For instance, it proves impossible to cut up such prepregs without a portion of the resin blowing about in the form of a large quantity of dry dust. Several approaches for modification of epoxy backbone for enhancing the properties of epoxy resins have been reported.
TW patent No. 293831 and WO 2006004118A1 disclose an epoxy resin composition including an amino-triazine-novolac (ATN) resin used as curing agent for epoxy resin. The amino-triazine-novolac resin can be represented by the general formula given below:

The amino-triazine-novolac resin was prepared by reacting a phenolic compound, an aldehyde compound and a guanamine compound in the presence of an acid catalyst such as oxalic acid or p-toluene sulfonic acid. The phenolic compound used includes, for example, phenol, resorcinol, and alkyl phenols such as cresol and xylenol. The aldehyde compound used includes, for example, formaldehyde. The guanamine compound can be represented by the general formula given below:
where R is amino, phenyl, or alkyl such as methyl.
The guanamine compound used includes, for example, melamine, benzoguanamine, and methyl guanamine.
When the amino-triazine-novolac was used as curing agent for epoxy resin, it could improve the heat resistance of the copper-clad laminate made from such epoxy resin, and the peeling strength of the copper foil to the laminate base. Also, the amino-triazine-novolac resin, which is a phenol-aldehyde resin, can react well with the epoxy resin used for the fabrication of the copper-clad laminate. However, the copper-clad laminate using amino-triazine-novolac resin as curing agent is not easily to be processed, and has unsatisfactory electrical performance.
Therefore, various combinations of curing agents have been proposed in order to mitigate the disadvantages for using just a single curing agent. U.S. Pat. No. 6,716,530 disclosed a resin composition that included maleimide compounds and an amino-triazine-novolac resin used as composite curing agent so that the PCB manufactured by using such a resin composition could have high flame retardancy, but the manufactured PCB had some disadvantages like low breaking tenacity, and poor heat resistance and chemical resistance, which do not meet the requirements for PCB product.
Furthermore, the amino-triazine-novolac resin and dicyandiamide (DICY), and the amino-triazine-novolac resin and diaminodiphenylsulfone (DDS) serving as composite curing agent for an epoxy resin were respectively proposed by the present inventor in TW 099112368 and TW 099112372, and by using the two specially designed composite curing agents for the epoxy resin, the copper-clad laminates prepared are excellent in breaking tenacity, dielectric properties, and peeling strength of copper foil.
However, there still exists a continuing need for a new and improved composite curing agent that will improve the processibility and the dielectric properties of the epoxy resin used for the fabrication of the copper-clad laminate or PCB while rendering the epoxy resin to have high glass transition temperature, high heat resistance, high chemical resistance, high peeling strength between a substrate and a copper foil, and low water absorption.